Creating a 3D printable 2- part mold which are separable for a curved shell
I posted this similar query in another forum and got some pointers and so I am sharing it here for the benefit of other users. You can read the entire post here but I am summarising it below.

I used the edge vertices to make a land area and then extruded in opposite direction to make a mold. For the other side, I first  extruded the edges in upward direction and then translated the entire mold by same units and repeated the same in other direction.

I am not sure if the final result is printable. But I got close to what I was trying to achieve.